1. Field of the Invention
The present invention relates to a barcode reader which scans and irradiates a laser beam from a scanning mirror onto a barcode symbol, and reads the written information from the reflected light, and more particularly to, improvement of a laser scanner which scans a laser beam.
2. Description of the Related Art
Generally, a well-known barcode symbol consists of multiple bars of different line widths arranged with spaces to express various information according to the pattern of arrangement. Such a barcode symbol is printed on an article, or a printed sticker of a barcode is stuck to an article. A laser beam is scanned and irradiated to a barcode, and the reflected light is optically read and converted into an electric signal, and decoded as information.
Conventionally, as a means for scanning a laser ban, a polygon mirror is rotated with a motor and scanned. This requires a motor driving system, and consumes much electric power. Thus, generally, it is built in a stationary barcode reader, and when reading a barcode, a barcode symbol stuck to an article is brought to the area where a laser beam is irradiated. However, actually, a barcode is sometimes printed on a large heavy article, and a handy barcode reader is demanded, and a compact polygon mirror is developed.
To make a barcode reader portable, a grip is necessary to hold the unit by hand, and the unit must be compact and light with lower power consumption. Particularly, when considering a cordless unit, a problem arises in improvement of a laser beam scanning unit.
Thus, for example, the Jpn. Pat. Appln. KOKAI Publication No. 2001-076085 discloses a laser beam scanning unit, which scans a laser beam by reciprocating a scanning mirror, instead of a conventional polygon mirror. In this laser beam scanning unit, a scanning mirror is supported rotatable, and a permanent magnet is fixed by an arm to the scanning mirror. A coil is provided near the permanent magnet. By reciprocating the coil energizing direction, a magnetic field of the permanent magnet is formed to generate an attractive force or a repulsive force, and the scanning mirror is reciprocated by these forces. A leaf spring (an elastic member) is provided to limit the range of reciprocating the scanning mirror, and energize the mirror in the direction to return essentially to the center position or to promote the reciprocation.
With the above-mentioned structure, the laser beam scanning unit is made compact and light with lower power consumption, and the barcode reader can be made compact and light, realizing long time continuous operation.
When a handy type barcode reader is actually used, a barcode is read at a location where an article to be read information is placed. For example, it is necessary to read a barcode in a warehouse with high temperature and humidity or in a freezer containing frozen foods. Namely, a barcode reader should work correctly even in high-temperature high-humidity conditions to very-low-temperature conditions.
However, the laser beam scanning unit of the barcode reader disclosed in the Jpn. Pat. Appln. KOKAI Publication No. 2001-076085 uses one of resin, rubber and elastomer as an elastic member of a leaf spring, which limits the scanning mirror reciprocating range, and energizes the mirror in the direction to return essentially to the center position or to promote the reciprocation. Particularly, the mechanical characteristics of the elastic member made of resin are highly dependent on temperature, especially the temperature-dependence of the modulus of elasticity may increase the driving energy at a low temperature or change the scanning angle as a result of the increased driving energy. This suggests that when the laser beam scanning unit is reciprocated with the eigen frequency f0, the eigen frequency f0 is temperature dependent. When the unit is reciprocated with the eigen frequency f0, the energy consumption can be minimized, and this is useful for a handy type barcode reader driven by a battery power. However, on the other hand, the eigen frequency f0 controls the laser scanning speed, and the temperature dependence of the eigen frequency is desirably low to obtain the stable barcode reading. performance always at every temperatures. Further, the change of the scanning angle causes a change of the laser scanning angle, and affects the barcode reading performance.
With a handy type barcode reader, the operator may accidentally drop a barcode unit on the floor, and durability against the impact of dropping is required.
Further, the required performance of a laser beam scanning unit is different in the scanning mirror and the support member, and the unit is constructed with a plurality of parts which are different in the manufacturing method and characteristics. Therefore, the unit is constructed in one body by a certain assembly method, such as gluing, and the assembly error in the component parts have a large influence upon the laser beam scanning stability and optical accuracy. That is, not only the accuracy of the finished parts, but also the uniformity and skill in the assembly work are demanded.